Switched Mode Power Supplies (SMPSs), such as those used in computer systems, receive an alternating current (AC) voltage signal that has a sinusoidal waveform at a fundamental frequency. Generally, for reasons described more fully below, SMPSs supply an AC load current signal that does not have a sinusoidal waveform. These non-sinusoidal waveforms include harmonics at multiples of the fundamental frequency and cause harmonic distortion to occur. A harmonic at twice the fundamental frequency is called a second harmonic and a harmonic at three times the fundamental frequency is called a third harmonic.
When the current waveform is not sinusoidal the power factor of a power supply is less than 100%. A typical SMPD has a power factor of around 60%, therefore having considerable odd-order harmonic distortion (sometimes with the magnitude of the third harmonic as large as the magnitude of the fundamental).
The presence of third harmonic distortion causes an increase in operating temperature of the generation facility, which reduces the life of equipment including rotating machines, cables, transformers, capacitors, fuses, switching contacts, and surge suppressors. Third harmonic distortion also causes additional losses and dielectric stresses in capacitors and cables, increasing currents in windings of rotating machinery and transformers and noise emissions in many products, and bring about early failure of fuses and other safety components.
Power factor correction devices may be provided with active or passive components. Typically these devices use a bulk capacitor to supply current when the value of a sinusoidal voltage waveform is low to provide a nearly constant level of power at a fixed direct current (DC) voltage level.